The present invention relates to the processing of thin films, such as those used in the processing of very small structures including microelectronic devices.
As the size of microelectronic devices is reduced from one generation to the next, the demands placed on photolithography become ever greater. One consequence of the increased demands is the need to reduce the thickness of polymer films used as anti-reflective coatings (ARCs) and photoresist imaging layers. The demands are especially severe for certain types of applications such as first minima ARCs, thin imaging layers for bi-layer and tri-layer lithography schemes, and thin imaging layers used in photomask production.
One of the major problems associated with the use of thin polymer films for these applications has been the appearance of dewetting defects including “pinhole” defects. Dewetting defects usually occur due to long-range van der Waals forces. Due to Van der Waals forces, localized thinning of a polymer film on a substrate occurs when the polymer film has insufficient thickness to overcome a tendency to dewet from the substrate. An example of this phenomenon is illustrated in FIG. 1 for a bottom anti-reflective coating (BARC) layer disposed on a substrate of silicon dioxide. FIG. 1 illustrates a free energy curve 10 for a BARC layer disposed on a substrate of silicon dioxide, and a second curve 12 being the second derivative of the free energy curve 10. The BARC layer becomes unstable and has a tendency to dewet catastrophically at a thickness (50 nm) below which the free energy curve 10 turns sharply lower and heads negative. Such catastrophic dewetting is referred to as spinodal dewetting. The location of the zero in the second curve 12 illustrating the second derivative of free energy indicates a crossover point at about 85 nm between a film that dewets spinodally below that thickness and dewets via nucleation and growth of holes above that thickness. As further shown in FIG. 1, as the overall film thickness is increased, the free energy of the film passes through a maximum and starts to decrease slowly as the film thickness continues to be increased. In the thickness regime just beyond the thickness at which the film spinodally dewets, the film is metastable and can dewet via nucleation and growth of holes. If the film thickness at some localized point in the film falls below the 85 nm thickness of the crossover point, the film becomes locally unstable and dewets spinodally. A BARC film having a thickness of less than or equal to 80 nm, which is less than the crossover point thickness of 85 nm, is highly unstable, and dewets spinodally, rapidly dewetting to droplets. On the other hand, a BARC film having a thickness of 110 nm, does not dewet spinodally, but dewets locally via nucleation and growth of holes. However, a BARC film having a thickness of 200 nm, is so far from the crossover point on the free energy diagram that random local fluctuations in film thickness no longer result in local instability of the film.
Heretofore, there has been no known solution to this problem other than to increase the thickness of the film that is prone to dewet. Unfortunately, increasing the thickness is not permitted, because advanced lithography processes call for reductions rather than increases in film thicknesses, for the following reasons. A thickened BARC film unnecessarily increases the difficulty of etching through the BARC film. A thickened photoresist imaging layer also increases risk of line pattern collapse, as well as reduces the process window.
Currently, it is common to utilize surface treatments such as an hexamethyldisilazane (HMDS) prime, prior to forming a coating such as an ARC or a photoresist. Such treatment promotes adhesion by changing the surface tension, and can also affect wettability of the coating by changing the spreading coefficient. However, even when a coating has a positive spreading coefficient, pinholes can still form when instability is present due to long range van der Waals forces.